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- ItemStructural and chemical characterization of MoO2/ MoS2 triple-hybrid materials using electron microscopy in up to three dimensions(Cambridge : Royal Society of Chemistry, 2021) Frank, Anna; Gänsler, Thomas; Hieke, Stefan; Fleischmann, Simon; Husmann, Samantha; Presser, Volker; Scheu, ChristinaThis work presents the synthesis of MoO2/MoS2core/shell nanoparticles within a carbon nanotube networkand their detailed electron microscopy investigation in up to three dimensions. The triple-hybrid core/shellmaterial was prepared by atomic layer deposition of molybdenum oxide onto carbon nanotube networks,followed by annealing in a sulfur-containing gas atmosphere. High-resolution transmission electronmicroscopy together with electron diffraction, supported by chemical analysisviaenergy dispersive X-ray and electron energy loss spectroscopy, gave proof of a MoO2core covered by few layers of a MoS2shell within an entangled network of carbon nanotubes. To gain further insights into this complexmaterial, the analysis was completed with 3D electron tomography. By usingZ-contrast imaging, distinctreconstruction of core and shell material was possible, enabling the analysis of the 3D structure of thematerial. These investigations showed imperfections in the nanoparticles which can impact materialperformance,i.e.for faradaic charge storage or electrocatalysis.
- ItemMorphology, Optical Properties and Photocatalytic Activity of Photo- and Plasma-Deposited Au and Au/Ag Core/Shell Nanoparticles on Titania Layers(Basel : MDPI, 2018-7-6) Müller, Alexander; Peglow, Sandra; Karnahl, Michael; Kruth, Angela; Junge, Henrik; Brüser, Volker; Scheu, ChristinaTitania is a promising material for numerous photocatalytic reactions such as water splitting and the degradation of organic compounds (e.g., methanol, phenol). Its catalytic performance can be significantly increased by the addition of co-catalysts. In this study, Au and Au/Ag nanoparticles were deposited onto mesoporous titania thin films using photo-deposition (Au) and magnetron-sputtering (Au and Au/Ag). All samples underwent comprehensive structural characterization by grazing incidence X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Nanoparticle distributions and nanoparticle size distributions were correlated to the deposition methods. Light absorption measurements showed features related to diffuse scattering, the band gap of titania and the local surface plasmon resonance of the noble metal nanoparticles. Further, the photocatalytic activities were measured using methanol as a hole scavenger. All nanoparticle-decorated thin films showed significant performance increases in hydrogen evolution under UV illumination compared to pure titania, with an evolution rate of up to 372 μL H2 h−1 cm−2 representing a promising approximately 12-fold increase compared to pure titania.
- ItemGrain Boundary Phases in NbFeSb Half-Heusler Alloys: A New Avenue to Tune Transport Properties of Thermoelectric Materials(Weinheim : Wiley-VCH, 2023) Bueno Villoro, Ruben; Zavanelli, Duncan; Jung, Chanwon; Mattlat, Dominique Alexander; Hatami Naderloo, Raana; Pérez, Nicolás; Nielsch, Kornelius; Snyder, Gerald Jeffrey; Scheu, Christina; He, Ran; Zhang, SiyuanMany thermoelectric materials benefit from complex microstructures. Grain boundaries (GBs) in nanocrystalline thermoelectrics cause desirable reduction in the thermal conductivity by scattering phonons, but often lead to unwanted loss in the electrical conductivity by scattering charge carriers. Therefore, modifying GBs to suppress their electrical resistivity plays a pivotal role in the enhancement of thermoelectric performance, zT. In this work, different characteristics of GB phases in Ti-doped NbFeSb half-Heusler compounds are revealed using a combination of scanning transmission electron microscopy and atom probe tomography. The GB phases adopt a hexagonal close-packed lattice, which is structurally distinct from the half-Heusler grains. Enrichment of Fe is found at GBs in Nb0.95Ti0.05FeSb, but accumulation of Ti dopants at GBs in Nb0.80Ti0.20FeSb, correlating to the bad and good electrical conductivity of the respective GBs. Such resistive to conductive GB phase transition opens up new design space to decouple the intertwined electronic and phononic transport in thermoelectric materials.
- ItemCombined structural analysis and cathodoluminescence investigations of single Pr3+-doped Ca2Nb3O10 nanosheets(London : Nature Publishing Group, 2023) Changizi, Rasa; Zaefferer, Stefan; Ziegler, Christian; Romaka, Vitaliy; Lotsch, Bettina V.; Scheu, ChristinaDue to the novel properties of both 2D materials and rare-earth elements, developing 2D rare-earth nanomaterials has a growing interest in research. To produce the most efficient rare-earth nanosheets, it is essential to find out the correlation between chemical composition, atomic structure and luminescent properties of individual sheets. In this study, 2D nanosheets exfoliated from Pr3+-doped KCa2Nb3O10 particles with different Pr concentrations were investigated. Energy dispersive X-ray spectroscopy analysis indicates that the nanosheets contain Ca, Nb and O and a varying Pr content between 0.9 and 1.8 at%. K was completely removed after exfoliation. The crystal structure is monoclinic as in the bulk. The thinnest nanosheets are 3 nm corresponding to one triple perovskite-type layer with Nb on the B sites and Ca on the A sites, surrounded by charge compensating TBA+ molecules. Thicker nanosheets of 12 nm thickness (and above) were observed too by transmission electron microscopy with the same chemical composition. This indicates that several perovskite-type triple layers remain stacked similar to the bulk. Luminescent properties of individual 2D nanosheets were studied using a cathodoluminescence spectrometer revealing additional transitions in the visible region in comparison to the spectra of different bulk phases.
- ItemSpontaneous fluctuations in a plasma ion assisted deposition – correlation between deposition conditions and vanadium oxide thin film growth(Amsterdam [u.a.] : Elsevier, 2021) Frank, Anna; Dias, Miguel; Hieke, Stefan; Kruth, Angela; Scheu, ChristinaIn this work correlations between thin film crystallinity of plasma ion assisted electron beam evaporated vanadium oxide (VOx) and fluctuations of the deposition parameters during the growth process could be observed by in situ monitoring deposition conditions and electron microscopy studies. In the presented case, unintentional fluctuations in the gas flow at the plasma source caused by inhomogeneous melting of the target material lead to an increase in discharge current and therefore a decrease of the oxygen flow in the plasma source, resulting in the formation of highly crystalline bands due to a temporary increase in energy flux. The major part of the VOx thin film consists of a large number of nanocrystals embedded in an amorphous phase. In-depth structural analysis confirms a mixture of V2O5, in different modifications, VO2, as well as the mixed-valence oxides V4O9 and V6O13, for nanocrystalline parts and crystalline bands. These differ mainly in the degree of crystallinity being influenced by variations in discharge current, and partly in the amount of higher oxidized vanadium oxides. In future, precisely controlled variation of plasma source conditions will open up pathways to control and tailor crystallinity of electron beam evaporated thin films, allowing for production methods for patterned thin films or layers with graduated crystallinity. This may give rise to a new class of coatings of nanohybrids combining amorphous VOx with low electrical conductivity and crystalline domains providing a higher electrical conductivity which is useful for electrochromic displays, smart windows, and solar cells.